This invention pertains to thermal switches and the method of manufacture thereof as well as a tool usable in said method.
In most snap-acting bimetal disc type thermal switches, the snap action of the disc is coupled to the contact mechanism by an insulated coupling pin or plunger, commonly referred to as the striker pin. This pin is normally made from vitreous-type material. The length of this pin must be precisely controlled to properly couple the snap travel of the disc to the contacts. Incorrect pin lengths result in improper switch action and either gross reduction in switching life or susceptability to intermittent contact closings during vibration. Normal manufacturing tolerances do not allow this pin length to be controlled directly without extraordinarily tight controls on the several parts that make up the assembly. As a result, normal practice has been to manufacture the detail parts to common tolerances, and compensate for the total accumulation of plus and minus tolerances by use of a striker pin fitted to each specific application. Two common methods are now used to fit the pin length to each switch. Both have limitations and advantages. The new method proposed herein combines the advantages of both and eliminates their fundamental disadvantages.
The most commonly used current procedure utilizes a free floating coupling pin, manufactured in incremental lengths to cover all possible combinations of tolerance accumulations. Each switch-contact assembly is measured using specialized gauges, which relate the geometry of each assembly to a specific pin size. The specified pin length is selected from available stock and installed in the switch. Since the design approach does not attach the pin to any support, it is free to rattle and bounce within the enclosure, where contamination from rubbing surfaces can be generated. Vibration and shock exposures can also impact the floating pin on the contact assembly, causing inadvertent openings or closings of switch contacts. Fractures of the pin as a result of extreme shock and vibration levels have also been observed.
The other commonly used procedure for obtaining correct pin length has been to mechanically attach a pin of sufficient length to compensate for all combinations of detail parts to a fixed part of the assembly, and then trim it to the specific dimension required. This design provides superior resistance to high vibration and shock levels, since no "loose" parts are used in the disc-to-armature geometry. However, the trimming operation does, by its nature, create debris in the form of chips or grindings which have the potential for contaminating switch contacts. Elaborate procedures are sometimes required to be absolutely sure that switches are thoroughly cleaned.
In grinding of a pin to length, a flat lower end is formed which results in harmful abrasive wear of the actuator disc by repeated contact therewith. Additionally, the grinding leaves a sharp lower edge with the result that particles may break off during use to cause contamination.